An input signal obtained via an image sensor is converted at an A/D converter to a digital signal assuming a value equal to or less than a predetermined level. However, when a very bright image is captured, gradations equal to or higher than the maximum level cannot be expressed and, accordingly, knee processing is executed to compress data assuming a given gradation level and higher so as to express the gradations within a range equal to or less than the specific value. For instance, the levels of the individual color signals among three primary color signals having been input may be determined, knee processing may be executed on two specific signals based upon the signal levels and a specific type of processing may be executed on the remaining signal based upon the signal values indicated by the other two signals before and after the knee processing (see patent reference 1).    Patent reference 1: Patent Publication No. 3596387